This invention relates to a method of digital correlation and to a computer program, a computer-readable storage medium and apparatus for the same
The requirement to correlate two digital signals arises in many applications. For example, in a receiver for CDMA type communication including a global positioning system (GPS) receiver, it is necessary to correlate an incoming CDMA signal containing a pseudorandom noise code (PRN) with a locally generated replica PRN code in order to determine the code phase error between the two, and thereafter acquire and track the CDMA signal.
Such signals may be represented in the form of 1, 1.5, 2 or more bits. The more bits used, however, the more complex is the circuitry or, if implemented by software, the greater the microprocessing power required to process the signals. Known methods of digital correlation are disclosed, inter alia, in U.S Pat. Nos. 4,507,748, 4,593,378, 5,239,496 and 5,305,245.
It is an object of the present invention to provide an alternative, efficient method of digital correlation.
According to the present invention, there is provided a method of correlating first and second digital signals when both signals contain a series of numerical data values and apparatus having processing means for performing the same, the method comprising the steps of:
(a) sequentially calculating products of corresponding numerical data values of the first and second digital signals and providing a cumulative sum thereof;
(b) upon deviation of the cumulative sum from between upper and lower thresholds levels, increasing or decreasing the cumulative sum by a predetermined amount so as to return the cumulative sum to between said threshold levels; and
(c) providing a correlation parameter as a function of the number of occurrences of deviation of the cumulative sum from between upper and lower thresholds.
Such a method provides an alternative, efficient method of digital correlation.
In a preferred method, the correlation parameter either increases or decreases depending on whether the cumulative sum deviates above the upper threshold level or below the lower threshold level.
In such a method, the correlation parameter is conveniently provided as a counter wherein the counter increases by one when the cumulative sum deviates above the upper threshold level; and wherein the counter decreases by one when the cumulative sum deviates below the lower threshold level.
Where the cumulative sum is stored in a register, wherein step (b) may be accomplished by allowing the register to either underflow or overflow. This can be provided using a simplified processing architecture.
The first signal may be a lower bit representation of a third digital signal or alternatively, a low bit representation of an analogue signal, further simplifying processing.
Also provided in accordance with the present invention is a GPS receiver comprising receiver means for receiving a subject signal containing a target pseudorandom noise code; and processing means for generating early and late replica code signals corresponding to the target code, correlating the subject signal with the early and late replica code signals and returning respective early and late correlation values, and calculating a code phase discriminator for determining whether the target code has been acquired; wherein the correlation between the subject signal and the early and late replica code signals is performed by steps:
(a) sequentially calculating products of corresponding numerical data values of the subject signal and early and late signals and providing a cumulative sum thereof;
(b) upon deviation of the cumulative sum from between upper and lower thresholds levels, increasing or decreasing the cumulative sum by a predetermined amount so as to return the cumulative sum to between said threshold levels; and
(c) providing early and late correlation values as a function of the number of occurrences of deviation of the cumulative sum from between upper and lower thresholds.
Where the subject signal is received as a carrier wave signal modulated by the target code, the method may further comprise the step of providing in phase (I) and quadrature phase (Q) components of subject signal; wherein, the I and Q components are each correlated with the early (E) and late (L) replica code signals to provide respective IE, IL, QE and QL correlation values; and wherein the code phase discriminator is calculated as a function of IE, IL, QE and QL.
This provides enhanced code phase correlation in circumstances where there is no precise carrier phase lock.
Ideally, the early and late correlation values either increase or decrease depending on whether the cumulative sum deviates above the upper threshold level or below the lower threshold level. In such circumstances, the early and late correlation values may be counters wherein the counters increase by one when the cumulative sum deviates above the upper threshold level and decrease by one when the cumulative sum deviates below the lower threshold level.
Where the cumulative sum is stored in a register, wherein step (b) may be accomplished by allowing the register to either underflow or overflow.